Sample callback for Microsoft Windows

The following example is the C code for the VA Smalltalk window procedure for Microsoft Windows. This example demonstrates how to call back into Smalltalk. Similar code will also work under UNIX platforms.

Note:

VA Smalltalk supports external language entry points that can be used instead of hand-written code. See Entry points.

Important points about the C code are:

•	The call-in is not performed from a user primitive, so the EsDefineUserPrimitiveEnvironment macro must be used before any VA Smalltalk API calls are made.

•	The calls to EsSaveObject are necessary because the EsU32ToInteger, EsI64ToInteger, EsU64ToInteger and EsUPtrToInteger calls can result in a garbage collection.

•	The object result from EsSendMessage is a Smalltalk object, so it must be converted back to C format before being returned.